1. Field of the Invention
The present invention relates to a composite oil-ring composed of a pair of rail rings and an expander-spacer ring which is to be disposed in an oil-ring groove of a piston reciprocally positioned in a cylinder of an internal combustion engine.
2. Description of the prior Art
Conventionally, the composite oil-ring has been popularly used in a gasoline engine on account of its high bore-followability and high sealability in the oil-ring groove. When it is used in an engine burning a high-leaded gasoline, however, the composite oil-ring suffers heavy wear, and it has been common practice to cope with this problem by Cr-plating or tufftriding the especially vulnerable inner periphery of the rail ring and the expander-spacer ring.
Even the composite oil-ring thus treated will soon be worn out and lose its function of oil sealing, if it is used in a diesel engine. For this reason, the composite oil-ring to be used in a diesel engine has the inner periphery of its rail ring and its expander-spacer ring thickly Cr-plated (to a thickness 10-20 times greater than for a composite oil-ring used in an engine burning a high-leaded gasoline ) or thickly tufftrided (which requires use of a special material), but these measures will not help save the resources and energy and will even result in an increased cost.